Bridezilla mum!

Hi everyone, I need some advice on how to deal with my bridezilla mum, she is getting married to my soon to be step dad in September and is being very unreasonable! When she got engaged I was pregnant with my rainbow baby, my son, Jack - so she bought my bridesmaid's dress in a size 18, as my son would have been around 1 or 2 weeks old on the day of her wedding (I was a size 14 before I got pregnant and was at the time she bought the dress) obviously we were unsure of how big I'd be after having him so she bought that size, unfortunately my son passed away on the 11th of March due to a rare heart defect and he was born sleeping😔 my problem is, still having my baby weight is killing me, it upsets me so much thinking how I should still have my bump and my son should still be here, so I decided to make a change and loose my baby weight, only problem is - she's told me I need to put on more weight to fit into my dress as she refuses to exchange it for a smaller size, claiming that it's 'too much hastle' (I'm currently back to being a size 14) but I've been dieting and eating healthily for about two weeks now and have lost half a stone, I showed her my progress and instead of being supportive she went absolutely ballistic and said I was being selfish for loosing the weight and that I was deliberately trying to get out of being her bridesmaid because at her wedding was where all my family who live away from us would meet Jack - Ofcourse that day is going to be hard for me and my partner not having him there to meet everyone, but I wouldn't ever deliberately sabotage her day! Even when she first found out I was pregnant the first thing she said to me was 'well what about my wedding you've planned this so you can have all the attention' which I would never do! When he died she was constantly apologetic for about a week then started with this crap - I explained why I wanted to loose the weight and she said I should be over that by now and I need to grow up and do as she says so I don't ruin her day! - what should I do or even say to her I'm so sick of it! I will never get over what happened to my son and if she thinks otherwise then that just kills me inside, she should be supporting me not putting me down even more! What should I do😔 Thank you in advance for any advice it's much appreciated💙